Metal-alloy film resistor and method of making same



B. L. PHILLIPS METAL-ALLOY FILM RESISTOR AND METHOD OF MAKING SAME 5 Sheets-Sheet 1 N ENE INVENTOR BERNARD L. PHILLIPS on com a:

Filed March 23, 1966 265 0p 5d: L $3250.25 zoom 3 63 m @583 090% wm: 2 55. HIE Cw MEEEEEP zoom Eow Q2 2. 68 Sun? 9.03 35 2 5.3 0-9 .& J wZZ wP44Dwn u,Zw DDU4 9.00708 3 0m 53 Po. 3 E 23 96 tmonma Q6232 23.2; V 35 1t; wexozoz $3 36 wwEOm ATTORNEY METAL-ALLOY FILM RESISTOR AND METHOD OF MAKING SAME Filed March 25. 1966 B. L. PHILLIPS Aug. 19, 1969 5 Sheets-Sheet 4 I 50 PER CENT KARMA -mo mwCzuu mwmwmo mmm 203 22 mum mkmza v muzdpwawm m0 .PZEQEHEOQ mmDhdmmafimk PER CENT ALUM l N UM INVENTOR BERNARD L. PHILLIPS BY ATTORNEY United States Patent 3,462,723 METAL-ALLOY FILM RESISTOR AND METHOD OF MAKING SAME Bernard L. Phillips, Norwood, Mass., assignor to P. R. Mallory & Co. Inc., Indianapolis, Ind., a corporation of Delaware Filed Mar. 23, 1966, Ser. No. 536,738 Int. Cl. H01c 7/06, 7/00, 17/00 U.S. Cl. 338-7 14 Claims ABSTRACT OF THE DISCLOSURE The TCR characteristics in film resistors of the nickelchromium type and having an insulating layer of silicon monoxide are improved by applying aluminum to the resistive film.

This invention relates to metal-alloy film resistors with improved characteristics.

Resistors used in electrical and electronic circuits are of many varieties such as carbon composition, pyrolytic carbon, cermet, oxide film, metal film, and wire-wound. The need for such an array of types is a combination of cost, performance and, sometimes, size.

Generally, the resistor is expected to retain its initial value of resistance under various environmental conditions and electrical loading. Furthermore, no circuit noise should be introduced by the resistor and it should be nonreactive. In addition, it should be readily manufactured with an adequate yield that conforms to reliability specifications. Actually, there is no single type of resistor that meets all the pertinent specifications and so a trade-off between various characteristics and price is usually made to obtain essential characteristics at an economical price, and designing around the other less desired characteristics. The metal-alloy film resistor comes closest to exhibiting the best combination of desirable characteristics.

Metal film resistors have been made for several decades but only in the last decade have they been improved to the point where performance and reliability have been achieved at a relatively low cost. Other improvements remain to be made, however, and this present invention describes several such improvements.

The main object of this invention is to increase the yield of usable resistors produced.

A prime object of this invention is to reveal a type of construction of a film resistor whereby the temperature coeificient of resistance may be altered incrementally by successive layers of aluminum films applied over the resistive film.

Another object of this invention is to reveal a technique of fabricating a film resistor to obtain various temperature coefiicients of resistance, ranging from positive to negative values.

A further object of this invention is to reduce the resistance spread of each batch of resistors.

Another object of this invention is to reduce the total variation of temperature coefficient of resistance.

Othe objects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, wherein certain preferred embodiments are set forth for purposes of illustration. Like reference characters describe elements of similar function therein, and wherein the scope of the invention is determined rather than from the dependent claims.

The present invention, in another of its aspects, relates to novel features of the instrumentalities described herein for teaching the principal object of the invention and to the novel principles employed in the instrumentalities, whether or not the features and principles may be used in the said object and/or in the said field.

3,462,723 Patented Aug. 19, 1969 In general, a very important resistor characteristic is its temperature coetficient of resistance (TCR). Many factors influence that quality. Some are understood and can be controlled; others are not understood and, as a consequence, may not be controlled. It is known that a slow rate of deposition of the resistive film generally results in a more negative TCR than obtains for a rapid deposition. A thin film A.) will usually result in a more negative TCR than obtains from a thicker film.

Two alloy compositions in general use are Nichrome V and Karma. These alloys may be used in wire or ribbon form or the constituents may be mixed in powder form and evaporated. The deposited film composition will not be exactly the same as the original ingredients because of preferential evaporation, and reaction of residual gasses in the vacuum system, but some compensation may be effected.

Reference is made to the following drawings:

FIGURE 1 is a partially-sectioned longitudinal View of a typical film resistor.

FIGURE 1a is an enlarged view of a portion of the resistor of FIG. 1.

FIGURE 2 is a process chart showing the sequence of operations required to produce the film resistor shown in FIGURE 1.

FIGURE 3 is a partially-sectioned longitudinal view of an improved metal-alloy film resistor with an aluminum coated ceramic substrate.

FIGURE 3a is an enlarged view of a portion of the resistor of FIG. 3.

FIGURE 4 is a process chart showing the sequence of operations required to produce the film resistor illustrated in FIGURE 3.

FIGURE 5 is a partially-sectioned longitudinal view of an improved metal-alloy resistor with an integral nichrome-aluminum resistive film.

FIGURE 5a is an enlarged view of a portion of the resistor of FIG. 5.

FIGURE 6 is a process chart showing the sequence of operations required to produce the film resistor shown in FIGURE 5.

FIGURE 7 is a graph illustrating the relationship between TCR and Karma-aluminum ratio.

FIGURE 8 is a partially-sectioned longitudinal view of an improved metal-alloy resistor with an aluminum film overlay on the resistive film.

FIGURE 8a is an enlarged view of a portion of the resistor of FIG. 8.

FIGURE 9 is a process chart showing the sequence of operations required to produce the film resistor shown in FIGURE 8.

To understand the nature of the improvements constituting this invention, reference is made to the drawings in detail. These drawings represent suggestive embodiments of this invention and are intended to be illustrative or representative and not limiting in character. FIGURE 1 represents a typical film resistor consisting of a ceramic tube 10 with gold resinate bands 11 applied on the ends, a resistive coating 12, such as Nichrome V or Karma deposited over the outside of the tube 10 and also over the gold bands 11, an insulating coating 13 of silicon monoxide covering the resistive film 12 between the gold bands 11, gold-plated caps 14 with leads 16 attached to the outside end of the cap 14 and pressed over the gold band 11, and the assembled unit covered with an insulating plastic 15.

FIGURE 2 indicates the process steps involved in making such a resistor. The ceramic bodies are fed into a machine which applies a band of gold resinate to the ends of each body. The bodies are baked for about 15 minutes at about 350 C. until the resinate is dry enough to permit 3 handling. They are then baked again at 800 C. for about one hour to eliminate the organic binder, leaving the gold bands tightly adherent to the ends of the bodies.

The banded bodies are then placed within a vacuum chamber and heated to about 200-400" C. The chamber is charged with Nichrome wire wrapped around tungsten electrodes. The vacuum pump is started and the pressure within the chamber is reduced to about 2 1O torr. The tungsten filaments are heated and the Nichrome evaporated and deposited on the banded bodies for a predetermined time to obtain a specific value of resistance.

The vacuum chamber also has in it a furnace charged with silicon monoxide. Immediately after the Nichrome film is evaporated and deposited, the silicon monoxide is vaporized in a similar manner. Rotation of the units continues at the same vacuum of 2 5- torr and at a temperature of about 300-400" C. for about 45 minutes. This is referred to as vacuum annealing and improves the stability of the film. The units are cooled to room conditions and then stabilized by baking about 16 hours at 250 C. and cooling to room temperature. They are then placed in a helixing machine which cuts a helical groove through the film and slightly into the ceramic body. The resistance between terminals is continually measured as the helix is cut and the machine stops and discharges the resistor when the monitor indicates that a predetermined resistance value has been reached. End-caps, with attached leads, are pressed over the gold bands onto each end of the resistor and a thermosettingplastic cylinder is molded around the resistor with the leads protruding axially from the ends of the cylinder. The encapsulated units are placed in an automatic sorting machine which divides the resistors into various tolerance groups after which they are marked with identifying nomenclature and stored.

The temperature coefficient of resistance of such resistors may be expected to be :200 p.p.m./ C., depending on the rate of. deposition, the annealing time and temperature, stabilizing time and temperature and other factors.

It was found that a relatively minor change in making such a resistor could result in a vast improvement in the TCR. This improvement is shown in FIGURE 3 and incorporated in the process chart shown in FIGURE 4-. It consists of applying a thin aluminum film 17 upon the ceramic substrate before the resistive film is applied. This is done in the same vacuum chamber used to vacuummetallize the resistive material and silicon monoxide. Comparing FIGURE 4 with FIGURE 2, it will be seen that all operations prior to and after application of the aluminum film are the same in both cases. TCR values from about minus 300 to +200 p.p.m./ C. were obtained by varying the relative proportion of aluminum and Nichrome. It was found that resistors made this way had considerably narrower TCR ranges and that successive batches were virtually identical in characteristics. Also, the resistance variation was reduced.

For a given range of TCR, a wider range of resistivity may be utilized, and in some cases a given value of resistance after helixing can be produced from films having lower resistivity and thus possessed of greater thickness and improved reliability.

It was also found that similar results were obtained using the construction shown in FIGURE 5 and processed according to the technique shown in FIGURE 6. It will be noted that this technique is very similar to that previously described but aluminum is vaporized simultaneously with the Nichrome depicted as the resistive film 18. FIG- URE 7 is a graph showing a general relationship between the TCR and constant thickness films comprised of various ratios of aluminum to Karma. The TCR ranges from about minus 300 p.p.m./ C. to about +200 p.p.m./ C. for corresponding Karma values of 20% and 100% respectively.

Similar results were obtained by using the construction shown in FIGURE 8. Here, the aluminum film is vaporized 4 over the resistive film as shown in the process chart of FIGURE 9. The effect on the TCR is substantially the same as observed in the resistors depicted in FIGURES 3 and 5.

Summarizing, it has been shown that the use of an aluminum film vaporized over a ceramic substrate and overcoated with a Nichrome or Karma type of resistance film will modify the TCR of the resistor so that negative, zero, or positive values can be obtained. Substantially the same effect will be obtained by vaporizing simultaneously the aluminum and resistive material. Furthermore, similar results obtain when the aluminum film is deposited over the resistive coating.

In addition to obtaining a wide range of TCRs by the judicious use of this aluminum, the variation in TCR for each batch is substantially reduced. Also, the variation in resistance of the resistors so produced is much less, resulting in a greater yield of closer tolerance units.

Furthermore, if the TCR of a batch is found to be outside the specified range, additional coatings of aluminum with appropriate annealing may be performed to adjust the TCR value. Because of the serpentine characteristic of-the TCR-percent Karma curve as shown in FIGURE 7, such corrections may be readily made.

Having described this invention in detail, the following claims are made:

1. A thin film resistor comprising a refractory substrate, electrical conducting means disposed at the ends thereof, a resistive film consisting essentially of a nickel-chromium based alloy with aluminum applied to said film disposed on said substrate extending over said electrical conducting means, an insulative film consisting essentially of silicon monoxide disposed over said resistive film, electrically conductive cap means disposed at each end of said substrate, electrical leads extending from said cap means to form a'resistive unit, and insulating means substantially surrounding said unit, said leads extending through said insulating means.

2. A thin film resistor according to claim 1, wherein said substrate is a tube and wherein said electrical conducting means, said resistive film and said cap means substantially surround said tube.

3. A thin film resistor according to claim 1, wherein said aluminum is a thin film disposed between said substrate and said resistive film.

4. A thin film resistor according to claim 1, wherein said aluminum is a thin film disposed between said resistive film and said insulative film.

' 5. A thin film resistor according to claim 1, wherein said aluminum is alloyed with said resistive film.

6. A thin film resistor according to claim 1, wherein said electrical conducting means disposed at the ends of said substrate are gold resinate bands.

7. A thin film resistor according to claim 1, wherein said electrically conductive cap means are gold caps.

8. A method of improving the TCR characteristics of thin film resistors having a refractory substrate with a resistive film consisting essentially of a nickel-chromium based alloy deposited thereon and an insulative coating consisting essentially of silicon monoxide on said resistive film, comprising applying aluminum to said resistive film prior to applying said insulative film.

9. A method according to claim 8, wherein said aluminum is applied as a thin film between said resistive film and said substrate.

10. A method according to claim 8, wherein said aluminum is applied as a thin film between said resistive film and said insulative coating.

11. A method according to claim 8, wherein said aluminum is applied simultaneously with said resistive film as said resistive film is applied to said substrate.

12. A method according to claim 8, wherein a gold resinate band is applied at one end of said resistor between said substrate and said resistive film.

13. A method according to claim 11, wherein the ratio of said aluminum to said resistive film is from about 25-75 percent.

14. A method according to claim 8, wherein said film resistor is metallize'd at a pressure of about 2X10" torr at a temperature of from about 200-400 C.

References Cited UNITED STATES PATENTS 3,110,620 11/ 1963 Bertelsen 117-217 Wilson 29620 X Solow 291-620 X Cook 291-620 Triggs 338308 X Lood et a1. 29-620 Yournans 338-308' REUBEN EPSTEIN, Primary Examiner US. Cl. X.R. 

